If I were you I’d get a an i3-6100, 32gb of ram and a gtx 960 in a large tower desktop. In terms of overall bang for your buck that’s probably the best around imo and the large tower is just so much more stress free than trying to Macgyver it all into a mid.
To make some of the games the best possible thing would be to get some of the free scenes/plugins/assets and have him mess around with pre-existing assets. It will be so much easier to play around with things that already exist to get an idea for how things work holistically rather than trying to learn the quadrillion steps it will take to get there on your own. There’s no need to start from square one especially for beginners.
Starting with 3d modeling is a bad idea as it is going to be the most time consuming and he might get discouraged before ever really getting into the meat and potatoes of the Unreal Engine. 3d models are only one small part of the entire toolset.
Also it’s good to try out all the templates that come with UE4 by default, the flying, the fps, the 3rd person. Just messing around with the blueprints in those free templates will give him a handle on how everything works. At worst he’ll gain valuable tech experience and give it up.